This invention relates generally to the improvement of the functional properties or proteinaceous materials such as single-cell proteins, plant proteins, whey solids, and mixtures thereof. More precisely, this invention involves subjecting the protein-containing material to a controlled pH, temperature, and time treatment which results in the improvement of the functional properties. For purposes of this invention, yeasts are considered as being separate from the plant proteins and are included within the single-cell protein category.
In recent years much attention has been directed toward the development of protein materials which can be incorporated in foods or food additives suitable for human consumption. Looking at plant proteins available today, it has been observed that these materials contribute to the off flavor, after flavor, undesirable color, unbalanced nutrients, or unacceptability in various food products. Similarly, untreated single-cell protein materials have been observed to have adverse effects on dough property and the bread quality. As would be expected, mixtures of single-cell and plant protein material have undesirable functional characteristics from each of the separate protein source materials.
The use of single-cell materials as a source for protein and the problems associated therewith can be better understood by looking more closely at a member selected from this class of materials, such as yeast cells. Yeast cells have the characteristic flavor and aroma which are affected to some extent by the growth conditions and the after-harvest processing conditions. They have a complicated organoleptic profile which consists of both pleasing and unpleasant flavors. One of the reasons limiting the use of yeast materials in food systems is the deleterious effect of its "yeasty" flavor. Where it is desirable to use yeast material at high levels for protein enrichment, a product of bland taste is preferred. Although the majority of yeasty flavor components can be easily removed from the yeast cells by a hot water extraction method, the use of such a process results in the loss of 15 to 20% in product yield. Furthermore, the extracted cells will retain some bitter, beany, and metallic off-taste. The loss in yield may be compensated by the value of the meat-flavored extract as a by-product, but the poor flavor of the cell product would need definite improvement. In addition, the hot water-extracted yeast cells contain about 0.6 to 1.0% phosphorus and 0.01 to 0.02% calcium. In order to achieve a nutritional balance of the calcium-phosphorus ratio for a food system in which such yeast is used, additional calcium may be necessary.
Particular attention has been directed to the use of single-cell protein materials, such as yeast, as a replacer for egg solids and nonfat dry milk (NFDM). For example, in the bakery industry, 2 to 3% nonfat dry milk is normally used as an additive to improve the physical and nutritional quality of bread. However, in view of the increasing cost and decreasing availability of milk, many bakers are looking for a substitute. Although certan products derived from soy protein have gained some acceptance, the active search by food technologists for a suitable substitute for milk in food products continues.
In this regard we have observed that during the fermentation and baking of bread dough, the wheat protein (gluten) forms the structure to hold the small bubbles of gas which are generated. This functional property permits the bread to rise and results in the production of bread having good volume and fine crumb structure. However, when untreated single-cell materials, such as dried inactive yeast, are added to bread dough to replace 2% nonfat dry milk, undesirable changes are observed in the property of the dough which adversely affect the bread quality. Typically, dough which contains unreacted yeast is soft, stringy, sticky and moist to the extent of rendering it difficult to handle. In fact, the dough has poor machinability characteristics which are detectable from the mixing to the final proofing stage. The inferior property of the dough is probably due to the poor water absorption and the strong reducing property of the thiol group in the yeast cell which damages the gluten structure. We have now found that materials such as yeast, plant, whey solids and combinations thereof can be treated according to the process of this invention to yield products highly suitable for replacing egg solids and nonfat dry milk. During the treatment of the yeast cells in accordance with the present process, several things happen which improve the functional property of the cell. The yeasty off-flavor is greatly reduced and cell material becomes significantly bland in taste by heating the yeast cells under controlled pH reaction conditions. A large amount of buffering materials are released from the cell by the heating process, which increase the buffering capacity of the food system when they are incorporated as dry yeast cell material. The saponification of lipid material gives rise to a soap material which is a good emulsifier. Also, heating under alkaline pH conditions will enhance the auto-oxidation of the thiol groups and the water holding capacity.